This invention relates to a transmission of digital signals, e.g. in a digital radio communication system.
Iterative decoding algorithms have become a vital field of research in digital communications. The first discovered and still most popular encoding scheme suited for iterative decoding is the parallel concatenation of two recursive systematic convolutional codes, also referred to as xe2x80x98Turbo Codesxe2x80x99. The underlying xe2x80x98Turbo Principlexe2x80x99 is applicable more generally to other algorithms used in modern digital communications, and in the past few years, other applications of the xe2x80x9cTurbo Principlexe2x80x9d have been found.
Channel coding is used to make the transmitted digital information signal more robust against noise. For this the information bit sequence is encoded at the transmitter by a channel encoder and decoded at the receiver by a channel decoder. In the encoder redundant information is added to the information bit sequence in order to facilitate error correction in the decoder. For example, in a systematic channel encoding scheme the redundant information is added to the information bit sequence as additional, inserted xe2x80x9ccodedxe2x80x9d bits. In a non-systematic encoding scheme the outgoing bits are all coded bits, and there are no longer any xe2x80x9cnakedxe2x80x9d information bits. The number of incoming bits (information bits) at the encoder is smaller than the number of outgoing bits (information bits plus inserted coded bits, or all coded bits). The ratio of incoming/outgoing bits is called the xe2x80x9ccode rate Rxe2x80x9d (typically R=1:2).
Recent improvements using the xe2x80x9cTurbo Principlexe2x80x9d have shown that, in digital communication systems involving a plurality of users in wireless communication with a receiver, an improvement in the quality of the decoded signal can be achieved by applying iterative decoding steps to the received data. In particular, xe2x80x9cIterative Equalization and Decoding in Mobile Communication Systems"" by Baunch, Khorram and Hagenauer, EPMCC""97, pp 307-312, October 1997, Bonn, Germany, discusses the application of the Turbo principle to iterative decoding of coded data transmitted over a mobile radio channel.
In order to be suitable for iterative decoding, a transmitted signal must be encoded by at least two concatenated codes, either serially or parallelly concatenated.
FIG. 1 shows a serially concatenated coding scheme: the transmission is done on a block-by-block basis. The binary signal from the digital source is encoded firstly by an outer encoder and is then passed through an interleaver, which changes the order of the incoming bit symbols to make the signal appear more random to the following processing stages. After the interleaver, the signal is encoded a second time by an xe2x80x98inner encoderxe2x80x99. Correspondingly, at the receiver the signal is first decoded by the inner decoder in a first decoding step, deinterleaved, and decoded by the outer decoder in a second decoding step. From the outer decoder soft decision values are fed back as additional a priori input to the inner decoder. The soft decision values provide information on the reliability of the hard decision values. In a first iteration the decoding step is repeated and the soft decision values are used as input values for the first and second decoder.
The iterative decoding of a particular transmitted sequence is stopped with an arbitrary termination criterion, e.g. after a fixed number of iterations, or until a certain bit error rate is reached. It should be noted that the a priori soft value input to the inner decoder is set to zero for the very first decoding of the transmitted bit sequence (xe2x80x98Oth iterationxe2x80x99).
The inner and outer binary codes can be of any type: systematic, or non-systematic, block or convolutional codes. Simple mapping (e.g. antipodal or binary phase shift keying) is performed in the transmitter (after the inner encoder) and simple demapping is performed in the receiver (after the inner decoder) although for clarity this is not shown in FIG. 1. Likewise, FIG. 1 illustrates a single user scenario, although application of appropriate multiplexing provides a suitable multi user system.
At the receiver the two decoders are soft-in/soft-out decoders (SISO-decoder). A soft value represents the reliability on the bit decision of the respective bit symbol (whether 0 or 1 was sent). A soft-in decoder accepts soft reliability values for the incoming bit symbols. A soft-out decoder provides soft reliability output values on the outgoing bit symbols. The soft-out reliability values are usually more accurate than the soft-in reliability values since they are improved during the decoding process, based on the redundant information added with each encoding step at the transmitter. The best performance is achieved by a SISO-decoder which provides the A Posteriori Probability calculator (APP), tailored to the respective channel code. Several faster, but sub-optimal algorithms exist, e.g. the SOVA (soft output Viterbi algorithm).
In multilevel modulation, M bits (bit symbols) are grouped together at the transmitter to form one xe2x80x98mapped symbolxe2x80x99 (also briefly referred to as xe2x80x98symbolxe2x80x99). This symbol can be mapped onto a real or a complex signal space (i.e. real axis, or complex plane). The mapping operation simply associates the unmapped symbol (M bits, value from 0, . . . , 2mxe2x88x921) with a discrete amplitude level for Pulse Amplitude Modulation (PAM), a discrete phase level for Phase Shift Keying (PSK), any discrete signal point in the complex plane for quadrature Amplitude Modulation (QAM) or any combination of PAM, QAM, PSK. The mapping can be of any type.
At the receiver the incoming symbols are noise affected. The hard decision demapping operation associates the incoming symbol with the closest signal point in the signal space (signal point with minimum Euclidian distance in real or complex signal space) and takes for example the respective Gray-encoded codeword as the hard decision values (O,1) for the M bits per mapped symbol.
However, if multilevel modulation is used in conjunction with channel coding and soft channel decoding (i.e. a soft input decoder) the demapping operation preferably calculates soft reliability values as inputs to the channel decoder. For simplicity, the term xe2x80x9cmultilevel modulationxe2x80x9d is used when referring to PAM, PSK or QAM modulation, meaning xe2x80x98multi-amplitude levelxe2x80x99 for PAM, xe2x80x9cmulti phase levelxe2x80x9d for PSK, and xe2x80x9cmulti signal pointsxe2x80x9d for QAM.
In one prior proposal, apparatus for iteratively decoding a signal has a demapper which has a first input for receiving the signal and an output for generating a demapped signal; and a decoder which has an input for receiving the demapped signal and an output for generating a decoded signal, the demapper having a second input for receiving the decoded signal.
Each user in a mobile communication system may have a different Quality of Service (QoS) requirement, i.e. different BER and latency constraints due to differing communication services. For example: voice communication has the lowest BER requirements (i.e. can tolerate many bit errors) with the highest latency constraints (i.e. cannot tolerate long delays in two way conversation); visual communication has a higher BER requirement and high latency constraints; data communication (e.g. wireless Internet web-browsing) has the highest BER requirements and the lowest latency constraints. Each user communicates with the base station with a different signal quality (i.e. SNR), multipath propagation and fading due to differing distance from the base station, propagation environment and, if mobile, speed.
The mapping operation itself does not add redundancy (in contrast to the inner encoder in classic serially concatenated encoding schemes) to the signal, but links bits together by grouping several bit symbols to form one mapped symbol.
The demapper is a soft demapping device that has been modified in order to accept a priori information obtained from the decoder. The decoder is a channel decoder and can be any SISO-decoder (optimal APP, or other sub-optimal algorithm, e.g. SOVA). The iterative demapping and decoding can thus be regarded as a serially concatenated iterative decoding scheme whereby the inner decoder is replaced by the soft demapping device. The iterative demapping and decoding is stopped by an arbitrary termination criterion (e.g. after a fixed number of iterations, or when a certain bit error rare is reached).
Mutual information is a parameter taken from information theory, see xe2x80x9cThe Elements of Information Theoryxe2x80x9d by T. M. Cover and J. A. Thomas. It specifies the maximal possible throughput for a given communication channel condition.
If we regard the mapping/demapping as part of the communication channel we can describe the maximal throughput by means of unconditional bit-wise mutual information Io of the particular mapping.
The mean unconditional bit-wise mutual information Io for no other bits of the mapping known is defined as       I    o    =            I      ⁢              xe2x80x83            ⁢              (                              X            k                    ;          y                )              =                  1        M            ·                        ∑                      k            =            0                                M            -            1                          ⁢                  xe2x80x83                ⁢                  I          ⁢                      xe2x80x83                    ⁢                      (                                          X                k                            ;              Y                        )                              
where
M is the number of bits of the mapping (e.g. for a 16 QAM-mapping M=4, since 24=16)
Xk is the kth bit of the mapping, with k=0 . . . Mxe2x88x921; input variable to the communication channel
Y is the output variable of the communication channel;
I(Xk;Y) and thus Io are dependent on the applied mapping and the signal-to-noise ratio (typically given by Eb/No), and can be calculated according to methods given in xe2x80x9cElements of Information Theoryxe2x80x9d referred to above.
We have found by simulation, that the best mapping in an iterative demapping (IDEM) system depends on the Eb/No-region of interest and on the number of iterations Nblt that can be performed at the receiver (see FIG. 2). As explained before, this best mapping is determined by its unconditional bit-wise mutual information Io. Hence, for the optimal mapping we have
IO.opt=function of (Eb/NO,Nbit).
transmission system, comprising:
a coder for coding an input signal with an error checking or error correcting code, an interleaver for interleaving the bits of the coded signal, a mapper for mapping the interleaved coded signal into a multilevel signal, and a transmitter for sending the multilevel signal over a noisy channel in a transmission medium;
a receiver for receiving the multilevel signal distorted by noise (noisy multilevel signal), a demapper having a first input for receiving the noisy multilevel signal and operative to provide a soft demapped signal at an output, a deinterleaver, for deinterleaving the demapped signal, a decoder to decode the deinterleaved demapped signal to provide a soft decoded signal, an interleaver to interleave the decoded signal, the demapper having a second input for receiving the interleaved decoded signal and being operative to recalculate the demapped signal iteratively using the noisy signal and the soft decoded signal,
characterized by:
a mapping store for storing two different mappings having different mutual informations;
means for deriving an indication of the channel conditions;
means for selecting the mappings in a mixing ratio dependent on the derived indication of channel conditions, the mapper and demapper being operatively responsive to the selected mapping.
In accordance with another aspect of the invention, there is provided a digital transmission system, comprising:
a coder for coding an input signal with an error checking or error correcting code, an interleaver for interleaving the bits of the coded signal, a mapper for mapping the interleaved coded signal into a multilevel signal, and a transmitter for sending the multilevel signal over a noisy channel in a transmission medium;
a receiver for receiving the multilevel signal distorted by noise (noisy multilevel signal), a demapper having a first input for receiving the noisy multilevel signal and operative to provide a soft demapped signal at an output, a deinterleaver, for deinterleaving the demapped signal, a decoder to decode the deinterleaved demapped signal to provide a soft decoded signal, an interleaver to interleave the decoded signal, the demapper having a second input for receiving the interleaved decoded signal and being operative to recalculate the demapped signal iteratively using the noisy signal and the soft decoded signal,
characterized by:
a mapping store for storing two different mappings having different mutual informations;
means for determining the number of iterations to be carried out from a plurality of possible numbers of iteration; and
means for selecting the mappings in a mixing ratio dependent on the number of iterations as determined, the mapper and demapper being operatively responsive to the selected mapping.
In accordance with yet another aspect of the invention there is provided a digital transmission system, comprising:
a coder for coding an input signal with an error checking or error correcting code, an interleaver for interleaving the bits of the coded signal, a mapper for mapping the interleaved coded signal into a multilevel signal, and a transmitter for sending the multilevel signal over a noisy channel in a transmission medium;
a receiver for receiving the multilevel signal distorted by noise (noisy multilevel signal), a demapper having a first input for receiving the noisy multilevel signal and operative to provide a soft demapped signal at an output, a deinterleaver, for deinterleaving the demapped signal, a decoder to decode the deinterleaved demapped signal to provide a soft decoded signal, an interleaver to interleave the decoded signal, the demapper having a second input for receiving the interleaved decoded signal and being operative to recalculate the demapped signal iteratively using the noisy signal and the soft decoded signal,
characterized by:
a mapping store for storing two different mappings having different mutual informations;
means for deriving an indication of the channel conditions;
means for determining the number of iterations to be carried out from a plurality of possible numbers of iterations; and
means for selecting the mappings in a mixing ratio dependent on the derived indication of channel conditions and on the number of iterations as determined, the mapper and demapper being operatively responsive to the selected mapping.
Preferably the indication of the channel conditions is an estimate of the signal to noise ratio.
The soft output signals of the demapper and the decoder are preferably likelihood ratios, most preferably log-likelihood ratios.
Each mapping is preferably stored with an indication of its mutual information, and the system includes a mutual information store storing an approximation of the optimum mutual information with a plurality of respective combinations of channel conditions and number of iterations, the means for selecting being operative to identify an optimum mutual information from the number of iterations as determined and the nearest stored channel conditions to the derived indication, and to select the mapping in a mixing ratio to give a mutual information for the mix approximately equal to the optimum.
The selecting means is preferably operative to calculate the mixing ratio q according to the formula:   q  =                    I                  o          ,          max                    -              I                  o          ,          opt                                    I                  o          ,          opt                    -              I                  o          ,          min                    
where
Io,max is the larger mutual information of the stored mappings,
Io,min is the smaller mutual information of the stored mappings, and
Io,opt is the desired optimum mutual information.
The invention also extends to a method of transmitting a digital signal, comprising:
coding an input signal with an error checking or error correcting code, interleaving the bits of the coded signal, mapping the interleaved coded signal into a multilevel signal, sending the multilevel signal over a noisy channel in a transmission medium;
receiving the multilevel signal distorted by noise (noisy multilevel signal), demapping the noisy multilevel signal to provide a soft demapped signal, deinterleaving the demapped signal, decoding the deinterleaved demapped signal to provide a soft decoded signal, interleaving the decoded signal, recalculating the demapped signal iteratively using the noisy signal and the soft decoded signal,
characterized by:
storing a plurality of different mappings;
deriving an indication of the channel conditions; and
selecting the optimum mapping of the plurality dependent on the derived indication of channel conditions, the mapping and demapping being operatively responsive to the selected mapping and demapping.
The invention further extends to a method of transmitting a digital signal, comprising:
coding an input signal with an error checking or error correcting code, interleaving the bits of the coded signal, mapping the interleaved coded signal into a multilevel signal, sending the multilevel signal over a noisy channel in a transmission medium;
receiving the multilevel signal distorted by noise (noisy multilevel signal), demapping the noisy multilevel signal to provide a soft demapped signal, deinterleaving the demapped signal, decoding the deinterleaved demapped signal to provide a soft decoded signal, interleaving the decoded signal, recalculating the demapped signal iteratively using the noisy signal and the soft decoded signal,
characterized by:
storing a plurality of different mappings;
determining the number of iterations to be carried out from a plurality of possible numbers of iteration; and selecting the optimum mapping of the plurality, dependent on the number of iterations as determined, the mapping and demapping being operatively responsive to the selected mapping.
The invention yet further extends to a method of transmitting a digital signal, comprising:
coding an input signal with an error checking or error correcting code, interleaving the bits of the coded signal, mapping the interleaved coded signal into a multilevel signal, sending the multilevel signal over a noisy channel in a transmission medium;
receiving the multilevel signal distorted by noise (noisy multilevel signal), demapping the noisy multilevel signal to provide a soft demapped signal, deinterleaving the demapped signal, decoding the deinterleaved demapped signal to provide a soft decoded signal, interleaving the decoded signal, recalculating the demapped signal iteratively using the noisy signal and the soft decoded signal,
characterized by:
storing a plurality of different mappings;
deriving an indication of the channel conditions; means for determining the number of iterations to be carried out from a plurality of possible numbers of iterations; and
selecting the optimum mapping of the plurality dependent on the derived indication of channel conditions and on the number of iterations as determined, the mapping and demapping being operatively responsive to the selected mapping.